In the manufacture of solid state circuits and devices, it is often necessary to etch or deposit onto material that is exposed through openings in a mask. Whereas etching can be done by using chemical solutions, the spaces between the edges of adjacent openings have to be large enough to allow for the fact that material under the edges is undesirably removed, a phenomenon known as undercutting. This may be avoided by exposing the material in the openings of the mask to ions that are in a plasma derived by a phenomenon known as electron cyclotron resonance, ECR.
The ECR process can also be used with advantage in depositing metals, dielectrics and other substances on material exposed by the openings in a mask. In the ECR process, gas that will produce the desired ion or ions is introduced into a region of a chamber in which electromagnetic waves are propagated along the axis of a D.C. magnetic field. Any free electrons having some motion component perpendicular to the lines of flux of the field will tend to move in a circular path at a frequency determined by the strength of the magnetic field. If the electromagnetic waves have the same frequency, they impart energy to the electrons so as to cause them to dislodge electrons from atoms in the gas and form ions. Once started, the process is accumulative so as to increase the number of ions until a steady state is reached in which the number of electrons equals the number of ions and the ion generation rate equals the ion loss rate.
The electrons move along the lines of magnetic flux in a direction of the weaker field. By virtue of the electrostatic field thus created, ions tend to be pulled along the same direction.
A major problem with the ECR process is that there may be a spatial variation in the density of the ions arriving at the mask. Thus, for example, if corresponding parts of different devices are respectively exposed through openings in the mask, the amount of etching or deposition at different openings is different so that the devices may have different electrical characteristics.
This nonuniformity arises from the fact that the density with which ions are produced depends both on the intensity of the microwave illumination and on the degree to which the strength of the magnetic field approaches the optimum value required for resonance and the fact that neither is uniform across the plasma source region. Since the electrons and ions tend to follow magnetic flux lines from the source region to the substrate at which etching or deposition is to take place, the nonuniformity of the plasma across the source will be imaged onto the substrate.
One previous method for reducing the degree of nonuniformity in the flux of ions arriving at the substrate has been to use a smaller cross section source in which the microwave modes are carefully controlled. Among other problems, however, the smaller cross-section tends to reduce the ion flux at the substrate to a smaller value.
A second approach that is described in an article entitled "A Parametric Study of the Etching of Silicon in SF.sub.6 Microwave Multipolar Plasmas: Interpretation of Etching Mechanisms" published at pages 825-834 of the June, 1987 issue of the Japanese Journal of Applied Physics, 26(G), operates without an axial magnetic field guiding the plasma from the source region to the substrate. Instead, a secondary cusp-type magnetic field, typically created using an array of permanent magnets bounding the drift tube connecting the source region and the substrate, aids radial confinement of the plasma and allows some mixing of radial nonuniformities of the plasma.